Gray level method for slm-based optical lithography

ABSTRACT

An optical lithography system comprises a light source, a spatial light modulator, imaging optics and means for continuously moving a photosensitive substrate relative to the spatial light modulator. The spatial light modulator comprises at least one array of individually switchable elements. The spatial light modulator is continuously illuminated and an image of the spatial light modulator is continuously projected on the substrate; consequently, the image is constantly moving across the surface of the substrate. While the image is moving across the surface, elements of the spatial light modulator are switched such that a pixel on the surface of the substrate receives, in serial, doses of energy from multiple elements of the spatial light modulator, thus forming a latent image on the substrate surface. The imaging optics is configured to project a blurred image of the spatial light modulator on the substrate, enabling sub-pixel resolution feature edge placement.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. Non-Provisional applicationSer. No. 10/646,525 filed Aug. 21, 2003, which claims the benefit ofU.S. Provisional Application No. 60/406,030 filed Aug. 24, 2002,incorporated in its entirety by reference herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to the field of optical lithography, and inparticular to printing patterns on the following substrates: wafers;printed circuit boards; flat panel displays; masks; reticles; and platesused for the reproduction of magazines, newspapers and books.

2. Description of the Related Art

The semiconductor industry uses very expensive stepper tools forlithographic processing. Furthermore, very expensive reticles are usedin this processing—the cost of the reticles is sufficient to make lowvolume production of chips (such as custom ASICs) prohibitivelyexpensive. The semiconductor industry needs a lower cost lithographyprocess. Furthermore, every time the lithography pattern changes,several days or more are required to produce a new reticle. Thesemiconductor industry needs a lithography process which can quicklyaccommodate pattern changes.

The printed circuit board (PCB) industry has similar problems with itslithography processes. Furthermore, the substrates used in the PCBindustry undergo distortion during fabrication which limits the use ofhigh resolution lithography processing to small area substrates and theuse of steppers. A high resolution lithographic process is required forlarge PCB substrates in which the pattern can be quickly andeconomically adjusted to accommodate the distortions, where thedistortions vary from one substrate to the next.

U.S. Pat. Nos. 5,330,878 5,523,193 5,482,818 and 5,672,464 to Nelsondescribe a method and apparatus for patterning a substrate. Theapparatus uses a spatial light modulator (SLM), specifically the TexasInstruments deformable mirror device (DMD), in place of a reticle. TheDMD is an array of individually controllable reflective elements. Animage of the DMD is projected on the substrate by an imaging lens.Whether or not an individual element of the DMD reflects light into theimaging lens, such that it is projected on the substrate, is determinedby computer; thus the pattern projected on the substrate is computercontrolled and readily changed. Improvements are required to thisapproach in order to meet the high resolution and throughputrequirements of both the semiconductor and PCB industries. Furthermore,advancements are available to reduce the cost of the apparatus, whileincreasing the throughput and meeting the high resolution requirements.

SUMMARY OF THE INVENTION

The present invention provides an apparatus and method for patterningphotosensitive substrates. The apparatus includes a spatial lightmodulator (SLM), a light source for illuminating the SLM, imaging opticsfor projecting an image of the SLM on the substrate, and means formoving the image across the surface of the substrate. The SLM controlsthe pattern of light which reaches the substrate. The SLM comprises atleast one array of individually switchable elements—switchable betweentwo or more states. The SLM can be either a diffractive or atransmissive device. The light source can be a continuous light source,such as an arc lamp, LED or continuous laser; quasi-continuous laserscan also be used when the laser pulsing frequency is much higher thanthe switching frequency of the elements of the SLM. The means for movingthe image can be a stage on which either the SLM or the substrate ismounted. When the substrate is in the form of a flexible film orsimilar, it may be moved using a reel to reel mechanism. While the imageis moving across the surface of the substrate, elements of the spatiallight modulator are switched such that a pixel on the surface of thesubstrate receives, in serial, doses of energy from multiple elements ofthe spatial light modulator, thus forming a latent image on thesubstrate surface. The imaging optics can be telecentric.

In preferred embodiments the imaging optics is configured to project ablurred image of the spatial light modulator on the substrate, enablingsub-pixel resolution feature edge placement. The blurring can beimplemented by: adjusting the focus of the imaging optics; adjusting thenumerical aperture of the imaging optics; adding a diffuser between theSLM and the substrate; adding a microlens array between the SLM and thesubstrate; or a combination of the aforementioned.

In preferred embodiments the spatial light modulator is continuouslyilluminated, an image of the spatial light modulator is continuouslyprojected on the substrate, and the image is continuously moved acrossthe surface of the substrate.

In some embodiments the SLM comprises a multiplicity of area arrays. Thecorresponding imaging optics can be a single projection lens system, ora multiplicity of projection lens systems. In the case of the latter,the number of the area arrays is greater than the number of theprojection lens systems, and the number of projection lens systems ispreferably a submultiple of the number of area arrays. Furthermore, themultiplicity of area arrays can be arranged in a line, or they can bearranged in multiple lines where the placement of the arrays isstaggered from one line to another. The latter may utilize more of theimaging field of the projection optics, and can also result in a moreefficient exposure of the substrate—reducing the need for a serpentinemotion of the projected image of the SLM across the substrate duringexposure.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a schematic representation of an optical lithography tool witha movable substrate, in accordance with the invention.

FIG. 2 is a schematic representation of an optical lithography tool witha movable spatial light modulator, in accordance with the invention.

FIG. 3 is a schematic representation of an optical lithography tool witha flexible film substrate, in accordance with the invention.

FIG. 4 is a detailed schematic representation of a first embodiment ofthe optical lithography tool of FIG. 1, showing telecentric projectionoptics.

FIG. 5 is a detailed schematic representation of a second embodiment ofthe optical lithography tool of FIG. 1, showing a spatial lightmodulator with multiple area arrays and corresponding multiple sets ofprojection optics.

FIG. 6 is a detailed schematic representation of a third embodiment ofthe optical lithography tool of FIG. 1, showing a spatial lightmodulator with multiple area arrays and a single set of telecentricprojection optics.

FIG. 7 is a diagrammatic cross-sectional view through part of amicro-mirror array in accordance with the invention, showing arrayelements in ‘on’ and ‘off’ positions.

FIG. 8 is a plan view of a substrate showing a serpentine path that canbe followed by the projected image of a spatial light modulator in orderto expose the entire substrate surface, in accordance with theinvention.

FIG. 9 is a plan view of a substrate showing serpentine paths that canbe followed by projected images from each of a multiplicity of areaarrays that are used together to expose the entire substrate surface, inaccordance with the invention.

FIG. 10 is a diagrammatic representation of the process of forming alatent image, in accordance with the invention.

FIG. 12 is a graph showing instantaneous light intensity distributionsalong the line segment AB on the substrate of FIG. 10, at equally spacedtime intervals T/10, starting at T3.

FIG. 13 is a graph showing instantaneous light intensity distributionsalong the line segment AB on the substrate of FIG. 10, at equally spacedtime intervals T/10, ending at T4.

FIG. 14 is a graph showing the integrated dose distribution along theline segment AB on the substrate of FIG. 10, due to light exposurebetween times T3 and T4.

FIG. 15 is a graph showing the total dose distribution along the linesegment AB on the substrate of FIG. 10, due to light exposure betweentimes T1 and T7.

FIG. 16 is a diagrammatic representation of the process of forming alatent image including a first example of edge shifting by one half ofthe projected width of a mirror, in accordance with the invention.

FIG. 17 is a diagrammatic representation of the process of forming alatent image including a second example of edge shifting by one half ofthe projected width of a mirror, in accordance with the invention.

FIG. 18 is a diagrammatic representation of the process of forming alatent image including an example of edge shifting by one quarter of theprojected width of a mirror, in accordance with the invention.

FIG. 19 is a diagrammatic representation of the process of forming alatent image including an example of edge shifting by three quarters ofthe projected width of a mirror, in accordance with the invention.

FIG. 20 is a diagrammatic representation of the process of forming alatent image including an example of edge shifting in another directionby one quarter of the projected width of a mirror, in accordance withthe invention.

FIG. 21 is a graph showing the integrated dose distributions along theline segment AB on the substrates of FIGS. 10, 16, 17, 18 and 19.

FIG. 22 is a further diagrammatic representation of the process offorming a latent image, in accordance with the invention.

FIG. 23 is a diagrammatic representation of the substrate array of FIG.22.

FIG. 24 is a graph showing the integrated dose distributions along theline segments CD, EF, GH and IJ on the substrate of FIG. 22.

FIG. 25 is a diagrammatic representation of the process of forming alatent image including a further example of edge shifting, in accordancewith the invention.

FIG. 26 is a diagrammatic representation of the substrate array of FIG.25.

FIG. 27 is a graph showing the integrated dose distributions along theline segments KL, MN, OP, QR and ST on the substrate of FIG. 25.

FIG. 28 is a block diagram of an optical lithography system inaccordance with the invention.

FIG. 29 is a plan view of an arrangement of multiple area arrays inaccordance with an embodiment of the invention.

FIG. 30 is a schematic representation of another embodiment of theoptical lithography tool of FIG. 4, showing a light switching mechanism121 on the light path between the light source and the substrate.

FIG. 31 is a timing diagram of an optical lithography system with twospatial light modulators configured in serial on the light path, inaccordance with the invention.

FIG. 32 is a diagrammatic representation of the process of forming alatent image using an optical lithography system with two spatial lightmodulators configured in serial on the light path, in accordance withthe invention.

FIG. 33 is a timing diagram of an optical lithography system with aspatial light modulator and a light switching mechanism configured inserial on the light path, in accordance with the invention.

FIG. 34 is a schematic representation of an optical lithography toolwith light optics configured to overlap projected images of two areaarrays on the substrate surface, in accordance with the invention.

FIG. 35 is a timing diagram of the optical lithography system of FIG.34.

DETAILED DESCRIPTION

With reference to FIG. 1, optical lithography tool 100, which is anembodiment of the invention suitable for patterning a substrate 140mounted on a movable stage 150, is shown with a light source 110, aspatial light modulator (SLM) 120 and imaging optics 130. Coordinateaxes 160 are shown with the z and y axes in the plane of the figure andthe x axis perpendicular to the plane of the figure. The light paththrough the optical lithography tool is represented by rays 170. Thelight source 110 continuously illuminates the SLM 120. The light sourcemay comprise an arc lamp, continuous laser (solid state or gas), lightemitting diode (LED) or other type of continuous light source that hassuitable spectral properties for exposure of the substrate 140.Furthermore, a light source such as a quasi-continuous laser (a laserwhich is pulsed at MHz frequencies), may be suitable as a light sourcefor this invention—a critical criteria is that the pulsing frequency bemuch higher than the switching frequency for elements of the spatiallight modulator (typically 10⁴ Hz); in this case the illumination of theSLM by the light source is effectively continuous. The light source mayalso comprise optical components for increasing the intensity ofillumination and to improve illumination uniformity. These may includean elliptical mirror, both round and cylindrical lenses, and light pipesor fly's eye lens arrays. SLM 120 is one or more area arrays (generallyrectangular) of elements that act on the light beam from the lightsource. An image of the SLM is continuously projected onto the substrateby the imaging optics 130, which is also referred to as the projectionoptics. The elements can be individually switched between two or morestates, under computer control, so as to control the light amplitude inthe image. One embodiment of the invention includes an SLM which is anarray of mirrors or diffractive elements that can switch incoming lightrays between two or more angular states. A digital micro-mirror device(DMD), currently available from Texas Instruments, is an example of asuitable mirror-array that can switch between two angular states. Anexample of a diffractive SLM is the Grating Light Valve (GLV) currentlymanufactured by Silicon Light Machines. Other embodiments of theinvention include SLMs which are Liquid Crystal Display (LCD) devices.If the elements of the SLM are transmissive, rather than reflective,then the optics will need to be rearranged; such a rearrangement will beobvious to those skilled in the art. Imaging lens system 130 may containboth reflective and refractive elements, and is typically telecentric.Substrate 140 either includes a photosensitive layer, such as aphotoresist coating, or is itself a photosensitive material, such as asheet of photosensitive polyimide. The stage 150 may be of aroller-bearing or air-bearing design and may have height adjustment (inz-direction), tilt and rotation capabilities. These types of stages arewell know and commonly used in lithography systems. For simplicity ofillustration the substrate is assumed to be planar. However, theinvention will also work with other substrate shapes, such ascylindrical or spherical, along with a rotary rather than a planarstage.

With reference to FIG. 2, optical lithography tool 200, which is anembodiment of the invention suitable for patterning a static substrate140, is shown with a light source 110, SLM 120, a stage 250 on which theSLM is mounted, and projection optics 130. The method of operation isthe same as for optical lithography system 100, described above, exceptthat stage 250 moves SLM 120 during exposure while substrate 140 isstationary. Imaging lens system 130 and/or illumination source 110 canalso be attached to the stage 250 and move with the SLM.

With reference to FIG. 3, optical lithography tool 300, which is anembodiment of the invention suitable for patterning a flexible substrate340, is shown with a light source 110, SLM 120, a stage 250 on which theSLM is mounted, projection optics 130, and rotatable, spaced apart,axially parallel film drums 342 and 344. The photosensitive flexiblefilm substrate 340 is wrapped around and tensioned between film drums342 and 344 such that the film can be moved in the y direction(referenced to stationary coordinate system 160). Two modes of exposureare possible. In the first mode, stage 250 moves the SLM at constantspeed in the x direction while the substrate 340 is stationary. When anexposure pass is complete (for example, the edge of the substrate isreached), the film drums index the substrate forward in the y directionand the stage reverses direction for the next exposure pass. The resultis a serpentine exposure path similar to path 850 shown in FIG. 8, anddiscussed in more detail below. In the second mode the roles of thestage and film drums are reversed. While the stage is stationary, thefilm drums move the substrate at constant speed in the y direction untilthe edge of the exposure region is reached. The stage then indexes thesubstrate forward in the x direction and the film drums reversedirection for the next exposure pass. Again, this results in aserpentine exposure path. Furthermore, if the width of the area to beexposed on the substrate is less than or equal to the width of theprojected image of the SLM, then the stage can remain stationary, or beeliminated, and the film drums move the substrate at a constant speed,without the need to reverse direction. As in other embodiments, theprojection optics may be carried on the stage.

Referring to FIGS. 4, 5 and 6, different embodiments of opticallithography tool 100 (see FIG. 1) are shown in detail.

FIG. 4 is a schematic of a continuous direct-write optical lithographysystem with an arc lamp and a telecentric projection lens system.Continuous illumination from mercury arc lamp 410 is reflected fromelliptical reflector 411. Reflected light, as represented by light rays170, travels to a dichroic mirror 412, which reflects wavelengths usefulfor exposure of the substrate 140 (for example 350 nm-450 nm) and istransparent to other wavelengths. Light not reflected from the dichroicmirror is absorbed in illumination beam dump 413. Other types of lampscan be used, such as a Xenon arc lamp, depending on the exposurewavelengths and source brightness needed. A light pipe 415 is used toimprove the illumination uniformity, but could be replaced with a fly'seye lens array. A light pipe lens system 414, positioned before thelight pipe 415, is used to adjust the numerical aperture of theillumination system and to adjust the diameter of the light beam priorto entering the light pipe. Condenser lens system 416 captures lightexiting from the light pipe and modifies the beam shape and angle tomatch the requirements of the SLM 120. The condenser lens systemcontains an illumination aperture 417. The light pipe lens system andcondenser lens system are usually anamorphic and contain cylindricallens elements. The continuous illumination mercury arc lamp, ellipticalreflector, dichroic mirror, illumination beam dump, light pipe lenssystem, light pipe, condenser lens system and illumination aperturecomprise an embodiment of illumination source 110, as shown in FIG. 1.The SLM is one or more area arrays (generally rectangular) of smallmirrors that can switch between two or more angular states undercomputer control. At least one of the angular states reflects light raysfrom the illumination source into a telecentric projection lens system430 and at least one other angular state reflects light rays into an SLMbeam dump 480. A digital micro-mirror device (DMD), currently availablefrom Texas Instruments, is an example of a suitable mirror-array thatcan switch between two angular states. Mirrors in the “on” state in theSLM are imaged on the substrate by the telecentric projection lenssystem. Light reflected from mirrors in the SLM in the “off” statetravels to the SLM beam dump where it is absorbed. Further details ofthe operation of a SLM are provided below and in FIG. 7. The substrateeither contains a photosensitive layer, such as a photoresist coating,or is itself a photosensitive material, such as a sheet ofphotosensitive polyimide. The substrate is attached to stage 150, whichmoves continuously during exposure in straight-line segments in the x-yplane of stationary coordinate system 160. The numerical aperture of thetelecentric projection lens system is determined by a projection lensaperture 432, which is optically conjugate to illumination aperture 417.A double telecentric projection lens system is shown. However, a singletelecentric or non-telecentric projection system will also work. Atelecentric design is preferred because the magnification does notchange with substrate height, which simplifies calibration of thelithography tool for each substrate. The telecentric projection lenssystem is a type of projection lens system 130, as shown in FIG. 1. Thestage can move in a plane x-y and also in the z direction of thestationary coordinate system 160. The stage 150 can also have rotationand tilt capability; this may be required for proper substrate alignment(for example, when substrate flatness is an issue). Movement in the zdirection will either focus or defocus the projected image on thesubstrate. A substrate height measuring system 450, utilizing heightdetection medium 490, can be used to determine the z position of thesurface of the substrate 140. The height measuring system can beoptical, capacitance or air based. The preferred type is air. Focusingcan also be accomplished by moving either the SLM or projection lenssystem in the z direction.

FIG. 5 is a schematic of a continuous direct-write optical lithographysystem with an arc lamp, an SLM with multiple area arrays, and multipleprojection lens systems. The light source is arranged as described abovefor FIG. 4, except that a condenser lens system 516 and lens array 518capture light exiting from light pipe 415, so as to modify the beamshape and angle to match the requirements of the individual SLM areaarrays 520 through 524. The lens array maximizes the light intensity onthe individual SLM area arrays; the lens array is configured to matchthe arrangement of the SLM area arrays, which may be arranged in a line,multiple lines (see FIG. 29), or some other two dimensional arrangement.While not an essential component, incorporation of the lens array ispreferred. The lens array may comprise lenses arranged correspondinglywith the SLM area arrays; alternatively, the lenses in the lens arraymay be replaced with one or more diffractive elements. Light pipe lenssystem 414 and condenser lens system 516 are usually anamorphic andcontain cylindrical lens elements. The continuous illumination mercuryarc lamp 410, elliptical reflector 411, dichroic mirror 412,illumination beam dump 413, light pipe lens system 414, light pipe 415,condenser lens system 516 and lens array 518 comprise a type ofcontinuous illumination source 110 as in FIG. 1. Each individual SLMarea array 520 through 524 is a rectangular array of small mirrors thatcan switch between two or more angular states under computer control. Adigital micro-mirror device (DMD) currently available from TexasInstruments is an example of a suitable mirror-array that can switchbetween two angular states. Mirrors in the “on” state in SLM area array520 are imaged on the substrate 140 by projection lens 530; likewise forSLM area arrays 521 through 524 and their corresponding projectionlenses 531 through 534. Light reflected from mirrors in SLM area array520 in the “off” state travels to SLM beam dump 480 where it isabsorbed; likewise for SLM area arrays 521 through 524. Five each SLMarea arrays (520 through 524), projection lenses (530 through 534) andsubstrate height measuring systems (550 through 554) are shown in thisexample, but any number may be used. The projection lenses may containboth reflective and refractive elements, and are typically telecentric.The projection lens systems (any one of 530 through 534) may be the sameas the projection optics 130 in FIG. 1. Substrate 140 either contains aphotosensitive layer, such as a photoresist coating, or is itself aphotosensitive material, such as a sheet of photosensitive polyimide.The substrate is attached to stage 150, which moves continuously duringexposure in straight-line segments in the x-y plane of stationarycoordinate system 160. As in other embodiments, the imaging optics maybe carried on the stage.

FIG. 6 is a schematic of a continuous direct-write optical lithographysystem with a single telecentric objective lens system and a SLM withmultiple area arrays. The light source 610 is the same as the lightsource described in FIG. 5, and is configured so as to provideillumination to match the requirements of the individual SLM area arrays520 through 524. Each individual SLM area array is one or morerectangular arrays of small mirrors that can switch between two or moreangular states under computer control. A digital micro-mirror device(DMD) currently available from Texas Instruments is an example of asuitable mirror-array that can switch between two angular states.Mirrors in the “on” state in the SLM area arrays are imaged on thesubstrate 140 by telecentric projection lens system 630, Light reflectedfrom mirrors in the SLM area arrays in the “off” state travels to SLMbeam dump 480 where it is absorbed. Five SLM area arrays are shown inthis example but any number may be used. A double telecentric projectionlens system 630 is shown. However, a single telecentric ornon-telecentric projection system can also be used. A telecentric designis preferred because the magnification does not change with substrateheight, which simplifies calibration of the lithography tool for eachsubstrate. The telecentric projection lens system is a type ofprojection lens system 130, as in FIG. 1. The stage can move in a planex-y and also in the z direction of the stationary coordinate system 160.The stage 150 can also have rotation and tilt capability; this may berequired for proper substrate alignment (for example, when substrateflatness is an issue). Movement in the z direction will either focus ordefocus the projected image on the substrate. A substrate heightmeasuring system 450 can be used to determine the z position of thesurface of the substrate 140. The height measuring system can beoptical, capacitance or air based. The preferred type is air. Focusingcan also be accomplished by moving either the SLM area arrays 520through 524 or telecentric projection lens system 630 in the zdirection. Substrate 140 either contains a photosensitive layer, such asa photoresist coating, or is itself a photosensitive material, such as asheet of photosensitive polyimide.

Further to the lithography systems of FIGS. 5 & 6, other embodiments ofthe invention are envisaged which combine a SLM having multiple areaarrays with a submultiple number of projection lens systems. Forexample, a lithography system may have 6 SLM area arrays and 2projection lens systems, such that each projection lens system images 3different SLM area arrays at once. Furthermore, the number of projectionlens systems need not be limited to a mathematical submultiple—forexample, a lithography system may have 7 SLM area arrays and 2projection lens systems, such that a first projection lens system images3 SLM area arrays and a second projection lens system images theremaining 4 SLM area arrays. The configuration of these embodiments willbe apparent to those skilled in the art. Clearly, there are very manyfurther combinations of SLM area arrays and projection lens systemswhich follow this teaching and will be apparent to those skilled in theart.

With reference to FIG. 7, a partial cross-section of a SLM 720 is shown.Mirrors 721 are shown in the ‘on’ position and mirrors 722 are shown inthe ‘off’ position. Light rays 770 are reflected off the surface of themirrors 721, which are in the ‘on’ position, toward a substrate (rays771) and are reflected off the surface of mirrors 722, which are in the‘off’ position, toward a beam stop (rays 772). For example, referring toboth FIGS. 4 and 7, rays 771 travel through projection lens system 430and then to the substrate 140, whereas the rays 772 fall outside theacceptance aperture of projection lens system 430 and are collected bybeamstop 480. This is the preferred mode of operation, although, othermodes of operation may be considered. For example, the rays 772 couldfall partly within the acceptance aperture of projection lens system430, consequently an attenuated signal from the “off” state mirrorswould reach the substrate, which may be tolerable.

With reference to FIG. 8, an example is shown of a serpentine path 850that can be followed by the projected image of a SLM in order to exposethe entire surface of the substrate 140. The motion of the image is dueto an image movement mechanism. The substrate or the SLM can be mountedon the image movement mechanism. An example of a suitable mechanism is astage, such as shown in FIGS. 1, 2 and 3. In the case of a flexiblesubstrate a suitable mechanism is a pair of rotatable, spaced apart,axially parallel film drums, such as shown in FIG. 3. In the explanationthat follows a configuration of the lithography system in which thesubstrate is mounted on a stage is assumed. The following are shown:substrate 140, serpentine path 850, distance between straight linesegments on the path 851, substrate coordinate system 853 and stationarycoordinate system 860. The SLM is oriented in such a way that thecolumns of pixels in the projected image on the substrate are parallelto the straight-line segment portions of the serpentine path, which, forease of illustration, are parallel to the x-axis of stationarycoordinate system 860. A stage positions the substrate 140 such that thecenter of the projected image of the SLM is at the beginning of path850. In this example, at the beginning of path 850 none of the projectedimage of the SLM falls on the substrate 140. As the stage moves in the+x direction, referenced to stationary coordinate system 860, the centerof the projected image of the SLM moves in the −x_(s) direction,referenced to substrate coordinate system 853, and traces the firststraight section of the serpentine path. The exposure starts when theprojected image of the SLM falls on the substrate. The exposure stopsafter the projected image clears the edge of the substrate. The stagethen repositions the substrate in readiness to scan in the −x directionalong the second straight section of the path, which is separated fromthe first straight section by a distance 851 in the y direction, allreferenced to stationary coordinate system 860. This is repeated untilthe entire substrate is exposed. Clearly, the projected width of the SLMmust be greater than or equal to the distance 851 in order to expose thecomplete substrate. If only certain regions of the substrate need to beexposed, then it may be more efficient to execute a serpentine patternfor each individual region. Although a serpentine path is preferred,other paths could be used as long as they contained straight-linesegments for the exposure. It will be clear to those skilled in the artthat a serpentine path can also be achieved with a lithography systemconfiguration in which the SLM is mounted on a stage and the substrateis static.

With reference to FIG. 9, an example is shown of a set of serpentinepaths 950 through 954 that can be followed by the projected images of acorresponding set of SLM area arrays in order to expose the entiresurface of the substrate 140. The motion of the image is due to an imagemovement mechanism. The substrate or the SLM can be mounted on the imagemovement mechanism. An example of a suitable mechanism is a stage, suchas shown in FIGS. 1, 2 and 3. In the case of a flexible substrate asuitable mechanism is a pair of rotatable, spaced apart, axiallyparallel film drums, such as shown in FIG. 3. In the explanation thatfollows a configuration of the lithography system in which the substrateis mounted on a stage is assumed. Each SLM area array is oriented insuch a way that the columns of pixels in the projected image on thesubstrate 140 are parallel to the straight-line segment portions of theserpentine path, which for ease of illustration, are parallel to thex-axis of stationary coordinate system 860. A stage positions thesubstrate 140 such that the centers of the projected images of the SLMarea arrays are at the beginning of paths 950 through 954. In thisexample, at the beginning of paths 950 through 954 none of the projectedimages of the SLM arrays fall on the substrate 140. As the stage movesin the +x direction, referenced to stationary coordinate system 860, thecenter of the projected images of the SLM area arrays move in the −x_(s)direction, referenced to substrate coordinate system 853, and trace thefirst straight sections of the serpentine paths. The exposure along anypath starts when the projected image of the SLM area array falls on thesubstrate. The exposure stops along any path after the projected imageclears the edge of the substrate. After all exposures have stopped, thestage then repositions the substrate in readiness to scan in the −xdirection along the second straight section of the path, which isseparated from the first straight section by a distance 851 in the ydirection, all referenced to stationary coordinate system 860. If thisdoes not cover the entire substrate, then the stage moves in the ydirection, referenced to stationary coordinate system 860, by thedistance between paths 950 and 954, and the above procedure is repeated.Clearly, the projected width of the SLM arrays must be greater than orequal to the distance 851 in order to expose the complete substrate.Note that in this example the separation between consecutive paths 950,951, . . . 954 is twice the spacing 851; should the separation exceedtwice the spacing 851, then a serpentine motion with more straightsections can be employed. This explanation is relevant to the multipleSLM area array lithography systems of FIGS. 5 & 6, for which paths 950through 954 correspond to SLM area arrays 520 through 524.

Referring to FIGS. 1 and 8, patterns of elements in the “on” state thatcorrespond to features printed on substrate 140 must shift across theSLM 120 in such a way that they appear stationary, on average, to theconstantly moving substrate. If the stage 150 is moving at constantspeed v along one of the straight-line segments of serpentine path 850(the stage is moving in a patterning direction), then this isaccomplished by shifting the SLM pattern by one row at regular timeintervals, where the time interval T is given by:

$\begin{matrix}{T = \frac{pM}{v}} & (1)\end{matrix}$

where p is the row pitch of the elements (the Texas Instruments DMDmirrors have the same pitch for rows and columns) and M is themagnification of the projection lens system 130. As an example, theTexas Instruments DMD is available with a mirror pitch of 13.7 micronsand the minimum mirror cycle time is 102 microseconds. If the projectionlens system 130 has a magnification of 2.0, then the stage speed isapproximately 269 mm/s. If the dose delivered is inadequate to exposethe substrate or the stage speed required is beyond the capability ofthe stage system, then the actual mirror cycle time used may need to belonger. However, the mirror cycle time and stage speed must alwayssatisfy equation (1).

FIG. 10 illustrates the shifting of patterns on the SLM and thecorresponding image on the substrate. In this example, the substrate ison a stage and moves at constant speed in the x direction duringexposures. The following are shown, with reference also to FIG. 1: partof SLM 120, which is an array of elements 1000 with an area of 4 rows by6 columns; a corresponding part of substrate 140, which is an array ofpixels 1002 with an area of 4 rows by 6 columns; resultant image 1007with projected row pitch (width of a pixel) 1008. The resultant imageshows one possible latent image on the substrate due to completion ofthe entire series of exposures. The edge placement and corner roundingin a latent image will be discussed in detail below. “Snapshots” of thecorresponding parts of the SLM and substrate are shown at equally spacedtimes T1 through T7, where the time interval satisfies equation (1); theparts of the SLM and substrate are indicated in the figure by M and S,respectively. The SLM array 1000, the substrate array 1002 and theresultant image 1007 are drawn as if viewed from a position directlyabove them and looking down in the −z direction of stationary coordinatesystem 160. For ease of illustration, in each “snapshot” the SLM andsubstrate arrays are shown next to each other. The projected row pitch1008 in the resultant image 1007 is the row pitch in the SLM array 1000times the magnification of the projection lens system 130. However, forease of illustration, in each “snapshot” the SLM and substrate arraysare shown having the same size and orientation. The grid shown on thearrays 1000 and 1002, and the image 1007 is for reference only. A lightsquare in 1000 corresponds to an SLM element in the “on” state, while adark square corresponds to one in the “off” state. The light and darkareas in 1002 correspond to the states of the SLM elements for that“snapshot”. For example, at time T1 the substrate is receiving light atpixels located at R1C4 and R1C5 from mirrors in the SLM array atpositions R4C4 and R4C5 (where the nomenclature R1C4 represents thepixel/element at row R1 and column C4). At time T1 the bottom edge ofthe substrate array 1002 is aligned with substrate position coordinate1. At time T2 the substrate has moved by one row and the bottom edge ofthe substrate is now aligned with substrate position coordinate 2. Thetime elapsed between T2 and T1 satisfies equation (1). The particularfeature pattern used as an example in FIG. 10 is shown in its entiretyat time T4 on both the SLM and the substrate arrays. It can be seen thatthe edge of this feature pattern first appears at T1, scrolls across theSLM array 1000 between times T2 and T6 and has moved off the SLM array1000 by T7. On the substrate array 1002, the feature pattern does notappear to move. This can be most clearly seen at times T3 and T4.However, because the substrate is moving at constant speed while the SLMis stationary, the projected pattern does in fact move on the substrateby the projected row pitch 1008 between any two consecutive snapshottimes. Note that for ease of illustration the patterns shown on thesubstrate arrays 1002 do not show any blurring or optical interferenceeffects.

FIG. 11 shows the substrate array 1002 with a line segment AB positionedin the center of column C4. Light intensity and resultant dose profileswill be determined on the surface of the substrate array in the positionindicated by line segment AB. Note that the position of AB is such thatit crosses the “trailing edge” of the exposure pattern shown in FIG. 10.

The consequences of the movement of the projected pattern across thesubstrate surface during exposure will now be examined. FIG. 12 showsinstantaneous light intensity distributions on substrate array 1002 fromFIG. 10; the distributions are along the position of line segment AB asshown in FIG. 11. Note that in FIG. 12 the line segment AB is shown toextend from −2 to 1.5 on the abscissa. In FIG. 12, six distributions areshown at intervals of T/10 starting at T3 and then every T/10, where Tis defined in equation (1) above. The substrate is moving with constantvelocity. The abscissa represents substrate displacement x_(s) (as shownin FIGS. 8 and 9) measured in units of projected row pitch (as definedabove in reference to FIG. 10). The following are shown in FIG. 12:light intensity profiles 1200, 1201, 1202, 1203, 1204 and 1205; 50%light intensity marker 1209; 50% position marker 1210; and projected rowpitch 1215. The shape of light intensity profiles 1200, 1201, 1202,1203, 1204 and 1205 is shown as being Gaussian; however, the actualshape depends on details of the optics. The instantaneous lightintensity as a function of position on the substrate array 1002 at timeT3 is represented by light intensity profile 1200. Light intensityprofile 1200 is positioned such that the intersection of the 50% marker1209 on the abscissa corresponds to the boundary between rows R3 and R4on the substrate array. The region between −1 and 0 on the abscissacorresponds to R4 on the substrate array, the region between 0 and 1corresponds to R3 and the region between 1 and 2 corresponds to R2. Asthe stage moves substrate array 1002 in the +x direction, theinstantaneous light intensity profile advances across the substratearray in the −x_(s) direction. The light intensity profiles 1201, 1202,1203, 1204 and 1205 are for times T3 plus T/10, 2T/10, 3T/10, 4T/10 and5T/10, respectively. The light intensity profile advances across thesubstrate in the −x_(s) direction by one-half of the projected row pitchduring T/2. In this example, at T3 plus T/2 the elements in SLM array1000 switch from the pattern shown at T3 to the pattern shown at T4.Looking specifically at the array elements responsible for generatingthe light intensity profiles: the element at C4R4 switches from “on” to“off”, the elements at C4R3 and C4R2 remain “on” and the element at C4R1switches from “off” to “on”. The effect is to shift the light intensityprofile from the position of 1205 to a new position which is one timesthe projected row pitch in the +x_(s) direction.

FIG. 13 follows on from FIG. 12 showing the light intensity profiles forthe next period T/2. After the elements switch at time T3+T/2 the lightintensity profile moves from the position of 1205 (see FIG. 12) to thatof 1300 (see FIG. 13). As the stage continues to move the substratearray 1002 in the +x direction, the instantaneous light intensityprofile advances across the substrate array in the −x_(s) direction. Thelight intensity profiles 1301, 1302, 1303, 1304 and 1305 are for timesT3 plus 6T/10, 7T/10, 8T/10, 9T/10 and 10T/10, respectively. Lightintensity profile 1305 is at time T3+T, which is the same as time T4.The light intensity profile advances across the substrate in the −x_(s)direction by one-half of the projected row pitch during T/2.Consequently, the position of light intensity profile 1305 at T4 is thesame as for profile 1200 at T3.

FIGS. 12 and 13 have shown how the light intensity distribution variesover the time interval between T3 and T4. FIG. 14 shows the resultantdose distribution for the same position on substrate array 1002—alongline segment AB. The light intensity distributions 1200 and 1300 inFIGS. 12 and 13 are Gaussian with σ=0.43. One can see from FIG. 14 thatthe resultant dose profile 1401 has a similar form to the originalGaussian.

The following are shown in FIG. 14: resultant dose profile 1401, 50%resultant dose marker 1404, 50% position marker 1405 and projected rowpitch 1215. Because the elements in SLM array 1000 in FIG. 10 areswitched when the substrate array 1002 has moved by one-half of theprojected row pitch 1008, the 50% resultant dose marker 1404 intersectsthe resultant dose profile 1401 at position 1405, which is the same asposition 1210 in FIGS. 12 and 13. This is due to the symmetrical natureof the process shown in FIGS. 12 and 13. Other choices of elementswitching time, besides Tn+T/2 (where n=1, 2, 3 . . . ), could be used(such as Tn+T/5). The shape of the resultant dose profile would be thesame as 1401, but the 50% resultant dose location on the abscissa wouldbe shifted from 0. Clearly, modulating the switching time can be used tocontrol the position of printed pattern edges. However, it is preferredto keep the switching time constant. The shape of the dose distributionwill not usually be the same as the instantaneous light intensityprofiles. This means that the dose profile for edges parallel to thedirection of stage motion will differ from those that are orthogonal.Edges parallel to the direction of stage motion are not constantlymoving, consequently the dose profile on the substrate for such an edgewill be identical to its instantaneous light intensity profile.

Referring back to FIG. 10, the process of switching the elements in SLMarray 1000 at times Tn+T/2 (where n=1, 2, 3, . . . ) is repeated untilthe pattern has completely scrolled off the SLM array 1000, which inthis example is at time T7. Since the time interval between any twoconsecutive “snapshot” times is equal to T from equation (1), theswitching times are equal to (T1+T2)/2, (T2+T3)/2, (T3+T4)/2, (T4+T5)/2,(T5+T6)/2 and (T6+T7)/2 respectively. Because the dose is additive, thefinal dose profile along line segment AB will have the same shape asresultant dose profile 1401 in FIG. 14. FIG. 15 shows the total doseprofile 1501. The following are shown in FIG. 15: total dose profile1501, 50% total dose marker 1504, 50% position marker 1505, exposedregion 1506, unexposed region 1507 and projected row pitch 1215. It ispreferred to adjust the total dose so that, after development, the edgeof the printed feature is at the 50% position marker 1505. In whichcase, the region with more than 50% total dose is the exposed region1506, while the region with less than 50% total dose is the unexposedregion 1507. Under these conditions, the final developed pattern wouldbe similar to the resultant image 1007 in FIG. 10—light areas correspondto exposed regions 1506 and dark areas correspond to unexposed regions1507. Except for some corner rounding, all the pattern edges line upwith the reference grid. Slight changes in exposure dose will affectvertical and horizontal dimensions differently. This is a practicalproblem only if the slope of the light intensity profile at and around50% light intensity is not steep enough (a steep slope allows sufficientline width control, assuming reasonable exposure and processingvariations).

Consider a lithography tool as in FIGS. 1 and 10 described above. Thedose distribution along the x direction on the surface of the substrateis given by the following equation:

$\begin{matrix}{{D(x)} = {N{\int_{0}^{T}{{I_{w}\left( {x,t} \right)}{t}}}}} & (2)\end{matrix}$

where N is a constant, I_(w)(x,t) is the time dependent light intensitydistribution at the surface of the substrate, and time T satisfiesequation (1). If the substrate is moving at constant speed v, then thelight intensity for the moving substrate, I_(w), is related to the lightintensity for the substrate at rest, I, by:

I _(w)(x,t)=I(x+vt,t)   (3)

Between t=0 and t=T/2 the elements in the SLM are in one state and shiftat t=T/2 by one row, i.e.;

I(x,t)=I ₀(x) 0<t<T/2

I(x,t)=I ₀(x−pM) T/2<t<T   (4)

Where I₀(x) is the intensity distribution for a single SLM element withthe substrate at rest, p is the row pitch of the SLM array elements andM is the projection lens system magnification. Using equations (1), (3)and (4), equation (2) can be written as:

$\begin{matrix}{{D(x)} = {N\left\lbrack {{\int_{0}^{T/2}{{I_{0}\left( {x + {vt}} \right)}{t}}} + {\int_{T/2}^{T}{{I_{0}\left( {x + {vt} - {vT}} \right)}{t}}}} \right\rbrack}} & (5)\end{matrix}$

As an example, assume the distribution I₀(x) is Gaussian in form. Thenfor 10 rows of elements in the “on” state the intensity distribution atthe substrate would be:

$\begin{matrix}{{I_{0}(x)} = {\frac{1}{2.51\; \sigma}{\int_{0}^{10}{^{{{- {({x - y})}^{2}}/2}\sigma^{2}}{y}}}}} & (6)\end{matrix}$

where σ² is the variance.

Equations (5) and (6) are examples of the form of equations used tocalculate the dose distributions and intensity distributions shown inthe Figures.

In order to make fine adjustments to the location of feature edges, a“gray level” technique can be used. When such a technique is implementedon apparatus such as that shown in FIGS. 1 through 6, it is requiredthat the image of an individual element of the SLM produced by theprojection lens system must be “blurred” i.e. the element is not clearlyresolved. This “blurring” can be accomplished in various ways includingdefocusing, using a microlens array or a diffuser or, more commonly, byadjusting the numerical aperture of one of the lenses in the projectionlens system to decrease the resolution to the desired value. Thepreferred method is defocusing. The technique can be understood byreferring to FIG. 16.

FIGS. 16 through 19 illustrate examples of “gray level” edge shifting ona pattern edge that is orthogonal to the direction of substrate motionduring exposure; in these examples the substrate is assumed to be movingin the same direction at constant speed during exposure. FIGS. 16through 19 are very similar to FIG. 10. The significant difference isthe displacement of the “trailing edge” of the resultant image by afraction of a pixel; for example, examination of the “trailing edge” ofthe resultant image 1600 in FIG. 16 shows a displacement 1601 which is0.5 times the row pitch 1008. Note that for ease of illustration thepatterns shown on the substrate arrays 1002 do not show any blurring oroptical interference effects.

In FIG. 16 the sequence of patterns on SLM arrays 1000 are identical tothe patterns shown in FIG. 10 at times T1, T2, T3, T4 and T6. However,at time T5 the elements in SLM array 1000 at locations R3C2, R3C3, R3C4and R3C5 are in the “on” state in FIG. 16 and in the “off” state in FIG.10. Also, at time T7 elements in SLM array 1000 at locations R1C2, R1C3,R1C4 and R1C5 are in the “on” state in FIG. 16 and in the “off” state inFIG. 10. With reference to substrate section 1002 in FIG. 16, pixelsR4C2, R4C3, R4C4 and R4C5 are exposed at times T5 and T7, but not attimes T1, T2, T3, T4 or T6. All other rows of the pattern are exposedfor four time periods—for example, pixels R1C4 and R1C5 were exposed attimes T1, T2, T3 and T4, while pixels R2C2, R2C3, R2C4 and R2C5 wereexposed at times T2, T3, T4 and T5. The effect of the two time periodonly exposure in row R4 is to produce an edge displacement 1601 ofroughly 0.5 times the width of the projected row pitch 1008, as can beseen in the resultant image 1600.

The sequence of exposures in FIG. 17 produces an edge displacement 1701of roughly 0.5 times the width of the projected row pitch 1008, as canbe seen in the resultant image 1700. This resultant image 1700 isidentical to the resultant image 1600 in FIG. 16; however, the tworesultant images are produced with different sets of exposure patterns.The exposure patterns in the 2 figures are different at times T4, T5, T6and T7. These 2 examples are certainly not exhaustive. One can easilyimagine other sequences of exposure patterns that give the sameresultant image.

FIG. 18 illustrates a further example of “gray level” edge shifting, inthis example the trailing edge displacement 1801 is 0.25 times the rowpitch 1008. The sequence of patterns on SLM array 1000 shown in FIGS. 10and 18 at times T1, T2, T3, T4, T6 and T7 are identical. However, attime T5 elements in SLM array 1000 at locations R3C2, R3C3, R3C4 andR3C5 are in the “on” state in FIG. 18 and in the “off” state in FIG. 10.With reference to substrate section 1002 in FIG. 18, pixels R4C2, R4C3,R4C4 and R4C5 are exposed at time T5, but not at times T1, T2, T3, T4,T6 or T7. All other rows of the pattern are exposed for four timeperiods. The effect of the one time period exposure in row R4 at time T5is to produce an edge displacement 1801 of roughly 0.25 times the widthof the projected row pitch 1008, as can be seen in resultant image 1800.

FIG. 19 illustrates a further example of “gray level” edge shifting, inthis example the trailing edge displacement 1901 is 0.75 times the rowpitch 1008. The sequence of patterns on SLM array 1000 shown in FIGS. 19and 10 at times T1, T2, T3 and T4 are identical. However, at time T5elements in SLM array 1000 at locations R3C2, R3C3, R3C4 and R3C5 are inthe “on” state in FIG. 19 and in the “off” state in FIG. 10. At time T6elements in SLM array 1000 at locations R2C2, R2C3, R2C4 and R2C5 are inthe “on” state in FIG. 19 and are in the “off” state in FIG. 10. Also,at time T7 elements in SLM array 1000 at locations R1C2, R1C3, R1C4 andR1C5, are in the “on” state in FIG. 19 and are in the “off” state inFIG. 10. With reference to substrate section 1002 in FIG. 19, pixelsR4C2, R4C3, R4C4 and R4C5 are exposed at times T5, T6 and T7, but not atT1, T2, T3 or T4. All other rows of the pattern are exposed for fourtime periods. The effect of the three time period exposure in row R4 attimes T5, T6 and T7 is to produce an edge displacement 1901 of roughly0.75 times the width of the projected row pitch 1008, as can be seen inresultant image 1900.

FIG. 20 illustrates an example of “gray level” edge shifting on apattern edge that is parallel to the direction of substrate motionduring exposure; in this example the substrate is assumed to be movingin the same direction at constant speed during exposure. FIG. 20 is verysimilar to FIG. 10. The significant difference is the displacement of anedge of the resultant image by a fraction of a pixel; for example,examination of the edge of the resultant image 2000 in FIG. 20 shows adisplacement 2001 which is 0.25 times the column pitch 2003.

In FIG. 20 the sequence of patterns on SLM array 1000 shown in FIGS. 20and 10 at times T1, T2, T4, T5, T6 and T7 are identical. However, attime T3 elements in SLM array 1000 at locations R2C6, R3C6 and R4C6 arein the “on” state in FIG. 20 and in the “off” state in FIG. 10. Withreference to substrate section 1002 in FIG. 20, pixels R1C6, R2C6 andR3C6 are exposed at time T3 but not at times T1, T2, T4, T5, T6 or T7.All other pixels on the substrate array 1002 are exposed for four timeperiods. The effect of the one time period exposure in column C6 at timeT3 is to produce an edge displacement 2001 of roughly 0.25 times thewidth of the projected column pitch 2003, as can be seen in resultantimage 2000.

Further to edge displacements, using one or more pixel exposures near acorner will affect the degree of corner rounding. For example, withreference to resultant image 1007 in FIG. 10, exposures at R1C1 or atboth R1C2 and R2C1 will change the corner rounding at location R2C2.

The edge displacements shown in the resultant images of FIGS. 16 through20 are only approximate; the actual displacements will depend on thedetailed shape of the instantaneous light intensity distribution at theedges of the exposure patterns. A more accurate determination can bemade by using a slightly modified form of equation (5) for the dosedistribution, including the light intensity distribution appropriate tothe mirror section states for each half of the 7 time periods. Thismodified form of equation (5) was used to calculate resultant dosedistributions along the position of line segment AB on substrate array1002 (see FIG. 11) for the exposure pattern examples given in FIGS. 10,16, 17, 18 and 19. In these calculations it is assumed that theinstantaneous light intensity distribution shape is Gaussian with a σvalue of 0.43. These resultant dose distributions are shown in FIG. 21.

In FIG. 21 resultant dose profiles 2101, 2102, 2103 and 2104 correspondto FIGS. 10, 16, 18 and 19, respectively; resultant dose profile 2102also corresponds to FIG. 17. 50% position markers 2105, 2106, 2107, 2108are for dose profiles 2101, 2102, 2103 and 2104, respectively. Withreference also to the resultant images in FIGS. 10, 16, 17, 18 and 19,the regions between −1 and 0 and 0 and 1 on the abscissa in FIG. 21correspond to R4 and R3, respectively, in the resultant images. 50%position marker 2105 of resultant dose profile 2101 was calculated forthe example given in FIG. 10 and intersects the abscissa at 0. Thisresult is consistent with the resultant image 1007 shown in FIG. 10. 50%position marker 2106 of resultant dose profile 2102 was calculated forthe example given in FIG. 16 and intersects the abscissa at −0.5. Thisresult is in agreement with the value of edge displacement 1601. 50%position marker 2107 of resultant dose profile 2103 was calculated forthe example given in FIG. 18 and intersects the abscissa at −0.20. Thisresult is slightly different from the edge displacement 1801 value of0.25. 50% position marker 2108 of resultant dose profile 2104 wascalculated for the example given in FIG. 19 and intersects the abscissaat −0.80. This result is slightly different from the edge displacement1901 value of 0.75.

It should be noted that the examples given above are simplistic andignore interference effects from adjacent elements of the SLM, therigorously correct shape of the light intensity distribution, and thefinite contrast of the photosensitive substrate. In general, the correctdose for a particular edge displacement will need to be determinedexperimentally. However, once the relationship between dose and edgedisplacement is determined, the technique can be used to compensate formisalignment and distortion of the substrate, distortion and aberrationsin the projection lens system, and non-uniform illumination. Thistechnique could be used to relax the specification of the optics, thusreducing the cost of the optics.

The preferred SLM device is the two-state DMD from Texas Instrumentswhich has a rectangular array of mirrors—1024 mirrors wide by 768mirrors deep. The scan direction during exposure of the substrate ispreferably orthogonal to the 1024 width in order to minimize the numberof times the stage must reverse direction along its serpentine path (seeFIG. 8). Since the array is 768 rows deep, the exposure patterns willscroll across the array in 768 discrete steps and there will be 768opportunities to adjust edge locations using the “gray level” techniqueoutlined above. This allows for an edge placement resolution of1/768^(th) the size of the projected row pitch of the DMD in theresultant image. In practice, one rarely needs more than 1/32^(nd).Consequently, 32 equally spaced edge positions can be chosen and theextra resolution can be used to compensate for non-uniform illuminationof the substrate.

The minimum feature size that can be printed on the substrate depends onthe characteristics of the light intensity profile. This will beexplained with reference to FIGS. 22 through 27.

FIG. 22 illustrates another example of the shifting of patterns on theSLM and the corresponding image on the substrate. As in previousexamples, the substrate is on a stage and moves at constant speed in thex direction during exposures. The following are shown, with referencealso to FIG. 1: part of SLM 120, which is an array of elements 2200 withan area of 5 rows by 6 columns; a corresponding part of substrate 140,which is an array of pixels 2202 with an area of 5 rows by 6 columns;resultant image 2207 with projected row pitch (width of a pixel) 1008.“Snapshots” of the corresponding parts of the SLM and substrate areshown at equally spaced times T1 through T8, where the time intervalsatisfies equation (1). This figure is similar to FIG. 10.

FIG. 23 shows the substrate array 2202 with line segments CD, EF, GH andIJ positioned in the center of columns C2, C3, C4 and C5. Lightintensity and resultant dose profiles will be determined on the surfaceof the substrate array in the positions indicated by the line segments.Note that the positions of the line segments are such that they crossboth the “trailing edge” and “leading edge” of the exposure patternshown in FIG. 22.

FIG. 24 shows resultant dose distributions for the exposed substrate2202, as detailed in FIG. 22. A Gaussian shape with a σ value of 0.43 isassumed for the instantaneous light intensity distributions used toderive the resultant dose distributions. The following are shown in FIG.24: resultant dose profiles 2400, 2401, 2402 and 2403 along linesegments CD, EF, GH and IJ, respectively; 50% position markers 2405,2406 and 2407 corresponding to dose profiles 2401, 2402 and 2403,respectively; 50% position markers 2404 and 2408, both corresponding todose profile 2400; and projected row pitch 1215. Note that the linesegment CD is shown to extend from −2 to 6 on the abscissa; linesegments EF, GH and IJ extend over the same values on the abscissa, butare not shown so as to avoid cluttering the figure. The regions between−1 and 0, 0 and 1, 1 and 2, 2 and 3, and 3 and 4 on the abscissa in FIG.24 correspond to R5, R4, R3, R2, and R1, respectively, on the resultantimage 2207 in FIG. 22. If the total dose is adjusted such that the edgeof printed features is at the 50% position markers, which is preferred,than the final developed pattern would be similar to the resultant image2207 in FIG. 22. It should be noted that resultant dose profile 2400 inFIG. 24 never rises higher than about 70% of dose profiles 2402 and2403, and that the distance between the 50% position markers 2404 and2408 is slightly less than the projected row pitch 1008. Clearly, underthese conditions the minimum feature size is roughly the same as theprojected row pitch 1008. The “gray level” technique described earliercan be used to adjust the width of such a feature—for example,decreasing the total dose for pixel R4C2 in substrate array 2202 of FIG.22 will reduce the height of resultant dose profile 2400, whichdecreases the size of the printed feature. However, the featuredimension changes rapidly with small changes in dose near the top ofdose profile 2400. Furthermore, there is always some noise anduncertainty in the total dose which places a practical limit on thisapproach.

FIG. 25 illustrates a further example of the shifting of patterns on theSLM and the corresponding image on the substrate. As in previousexamples, the substrate is on a stage and moves at constant speed in thex direction during exposures. In FIG. 25 examples of “gray level” edgeshifting on various sizes of feature are shown, where the shifted edgesare orthogonal to the direction of substrate motion during exposure. Thefollowing are shown, with reference also to FIG. 1: part of SLM 120,which is an array of elements 1000 with an area of 4 rows by 6 columns;a corresponding part of substrate 140, which is an array of pixels 1002with an area of 4 rows by 6 columns; resultant image 2507 with projectedrow pitch (width of a pixel) 1008. “Snapshots” of the correspondingparts of the SLM and substrate are shown at equally spaced times T1through T7, where the time interval satisfies equation (1). This figureis similar to FIG. 10.

FIG. 26 shows the substrate array 1002 with line segments KL, MN, OP, ORand ST positioned in the center of columns C2, C3, C4, C5 and C6. Lightintensity and resultant dose profiles will be determined on the surfaceof the substrate array in the positions indicated by the line segments.Note that the positions of the line segments are such that they crossthe “trailing edge” and “leading edge” of the exposure pattern shown inFIG. 25.

FIG. 27 shows resultant dose distributions for the exposed substrate1002, as detailed in FIG. 25. A Gaussian shape with a σ value of 0.43 isassumed for the instantaneous light intensity distributions used toderive the resultant dose distributions. The following are shown in FIG.27: resultant dose profiles 2700, 2701, 2702, 2703 and 2704 along linesegments KL, MN, OP, OR and ST, respectively; 50% position markers 2710and 2716 both corresponding to dose profile 2704; 50% position markers2710 and 2713 both corresponding to dose profile 2703; 50% positionmarkers 2711 and 2714 both corresponding to dose profile 2702; 50%position markers 2712 and 2715 both corresponding to dose profile 2701;and projected row pitch 1215. Note that the line segment KL is shown toextend from −2 to 5 on the abscissa; line segments MN, OP, OR and STextend over the same values on the abscissa, but are not shown so as toavoid cluttering the figure. The regions between −1 and 0, 0 and 1, 1and 2, and 2 and 3 on the abscissa in FIG. 27 correspond to R4, R3, R2,and R1, respectively, on the resultant image 2507 in FIG. 25. If thetotal dose is adjusted such that the edge of printed features is at the50% position markers, which is preferred, than the final developedpattern would be similar to the resultant image 2507 in FIG. 25. Itshould be noted that resultant dose profile 2700 in FIG. 27 never riseshigher than about 45% of dose profile 2704, and therefore does notprint. Resultant dose profile 2700 is due to exposure by alternatingsingle adjacent pixels, as can be seen by investigating column C2 ofsubstrate section 1002 at times T2, T3, T4 and T5 in FIG. 25. This is incontrast to the example of FIG. 22 where a single pixel exposure createda dose profile that did print. With reference to FIGS. 25 and 27, thefeatures printed in columns C3, C4 and C5 are all roughly 1.5 times theprojected row pitch 1008 in width as can be seen, for example, byexamining the distance between 50% position markers 2711 and 2714 ofresultant dose profile 2702. It appears that when a feature is at somearbitrary location with respect to the projected SLM element grid thenthe minimum (practical) feature size is roughly 1.5 times the projectedrow pitch; this is in contrast to the minimum feature size of roughly1.0 times the projected row pitch seen for features located on theprojected SLM element grid—see FIGS. 22 and 24.

With reference to FIG. 28, a block diagram for an optical lithographysystem of the invention is shown. Design data, which resides on thedesign data storage device 2804, describes what the system should printand is input to the data preparation computer 2805 for translating intoa form suitable for the decompression electronics 2807. The datapreparation computer 2805 can also modify the data to compensate forpreviously measured substrate distortion. Substrate alignment system2803 can be used to measure the substrate distortion. The design data istypically in a CAD (Computer Aided Design) format or a mask standardformat such as GDSII. The design data storage device may be one or moretapes or disk drives. The data preparation computer can be anygeneral-purpose computer such as an IBM PC. After computation by thedata preparation computer, the data is stored on one or more fast diskdrives 2806. The preferred form of this data can be understood byreference to the resultant image in FIG. 19. The entire area of thesubstrate 140 is divided into small squares with a pitch equal to themagnified pitch of the SLM array 120, substrate array 1002 provides asmall-scale example. Each pixel in the array covering the substrate isassigned a dose value that is based on the feature pattern and a look-uptable value. The look-up table values are determined experimentally andtake into account the distortions and aberrations of projection lenssystem 130 and the illumination non-uniformity from the illuminationsource 110. As an example, dose values are derived based on the featurepattern of resultant image 1900, assuming 32 gray levels, where 31corresponds to 100% exposure. The following pixels will have a dosevalue of 31:

-   R1C4, R1C5, R2C2, R2C3, R2C4, R2C5, R3C2, R3C3, R3C4, R3C5    The following will have a dose value of 0:-   R1C1, R1C2, R1C3, R1C6, R2C1, R2C6, R3C1, R3C6, R4C1, R4C6    The following pixels will have a dose value intermediate between 0    and 31, based on the intended edge location 1901:-   R4C2, R4C3, R4C4, R4C5    For convenience, we will assign a value of 24 to the above. Next, a    look-up table is used to modify the dose values to account for    distortions, aberrations and illumination nonuniformity of the    system. Since the preferred SLM, the Texas Instruments DMD device,    can switch mirror states every 102 microseconds and has 1024 rows    and 768 columns, this means that the fast disk drives 2806 need to    deliver 1 row of 1024 pixels every 102 microseconds. With 32 gray    levels this is a data rate of roughly 6.3 megabytes/second. This    data rate is easily within present day capabilities of disk drive    arrays.

Again referring to FIG. 28, alignment of the substrate 140 to the stage150 and projection lens system 130 is determined by reflecting substratealignment system light 2892 off features on the substrate 140 intosubstrate alignment system 2803. The substrate alignment system ispreferably a “machine vision” system that compares arbitrary features onthe substrate to previously stored images or idealized images, such as across or a circle, in order to find a match. The substrate alignmentsystem light could come from illumination source 110 by way of SLM 120and projection lens system 130, or from an external source. Afterreflecting off features on the substrate the light could travel directlyto the substrate alignment system, as shown, or could first travelthrough the projection lens system (“through the lens” alignment). Thelight reflected off features on the substrate could also travel throughthe projection lens system, reflect off the SLM and then pass into thesubstrate alignment system. Stage metrology system 2802 receives stageposition information from stage position optical sensor 2891, which canbe based on laser interferometers or linear scales, and sendsinformation to control computer 2801. In turn, the control computersends signals to the stage x, y motors which then servo to the correctlocation. If edge blurring is accomplished by defocusing, which is thepreferred technique, then the control computer commands the stage toservo in z until a suitable gap value is achieved. The gap value ismeasured by the substrate height detector 450 by way of substrate heightdetection medium 490, which is preferably air. Other types of detectiontechniques, such as optical or capacitance, would also work. The gapvalue (defocus) is chosen to produce the desired amount of feature edgeblurring in the image projected onto the substrate. Constant servoing tomaintain this gap value is needed to compensate for local substrateheight variations. Rather than move the stage in the z-direction, itwould also be acceptable to move the projection lens system 130 or SLM120 in the z-direction instead. Next, the control computer commands thefast disk drives 2806 to send the first row of data to the decompressionelectronics 2807, which loads the first frame of mirror state data tothe SLM memory 2808.

To understand the function of the decompression electronics 2807 it isnecessary to first understand the requirements of the SLM 120. All ofthe mirrors in the SLM switch states at the same time. The states of allmirrors are individually determined by values stored in the SLM memory2808. Therefore, the requirement for the decompression electronics isthat it must load the entire SLM memory with new mirror-state valuesevery mirror clock cycle. For the Texas Instruments DMD device, this isevery 102 microseconds. The decompression electronics must translate thedose values for each image pixel into a sequence of mirror states thatshift with the moving substrate. A simplified example based on FIG. 19can illustrate how this could be accomplished. For any pixel in theresultant image 1900, 5 dose levels are possible due to the four mirrorclock cycles used to shift each row across the mirror section 1000. Forexample, pixel R4C2 in substrate section 1002 can be exposed at time T4,T5, T6 and T7, as can be seen by inspecting FIG. 19. The actual exposurewas only at times T5, T6 and T7 for this pixel. Any of the five possibleexposure sequences can be represented by a string of 0's and 1's thatcorrespond to the mirror state at the 4 exposure times. For example, forR4C2 the string would be 0111. A suitable set of 5 exposure sequenceswould be: 0000 0001 0011 0111 1111

There are other possible sequences that give the same dose, such as 1000rather than 0001. This degree of freedom can be used to compensate forillumination non-uniformity from illumination source 110. The doselevels that correspond to the exposure sequences are defined to be 0, 1,2, 3, and 4. Prior to the mirrors switching at time (T4+T3)/2, doselevels for all of the pixels in row 4 (R4) of SLM array 1000 are sentfrom the fast disk drives 2806 to the decompression electronics 2807.The sequences that correspond to each possible dose level are stored ina look-up table in the decompression electronics. Again using pixel R4C2as an example, its dose level would be 3 which corresponds to thesequence 0111. Starting in the state shown at T3, the SLM memory 2808would have a 0 state loaded for the mirror in the fourth row and secondcolumn, R4C2, of the substrate array 1000. After the mirrors switch tothe state shown at T4, the decompression electronics loads the SLMmemory with the second digit in the exposure sequence (1) in the thirdrow and second column, R3C2, of the substrate array 1000. The mirrorsswitch state at (T5+T4)/2. After the mirrors switch to the state shownat T5, the decompression electronics loads the SLM memory with the thirddigit in the exposure sequence (1) in the second row and second column,R2C2, of the substrate array 1000. The mirrors switch state at(T6+T5)/2. After the mirrors switch to the state shown at T6, thedecompression electronics loads the SLM memory with the fourth digit inthe exposure sequence (1) in the first row and second column, R1C2, ofthe substrate array 1000. The mirrors switch state at (T7+T6)/2. Theprincipal of operation is the same for the much larger Texas InstrumentsDMD array. The decompression electronics must contain a memory largeenough to hold a dose level code for each of the mirrors in the SLM anda look-up table. The decompression electronics also contains logiccomponents to handle the bookkeeping. Because all of the mirror valuesneed to be determined and loaded into the SLM memory during the 102microseconds mirror clock cycle, many mirror values need to be computedin parallel. For example, if it takes 100 nanoseconds to calculate thenext state for a single mirror, then the computations for roughly 800mirrors must clearly be done in parallel.

The control computer 2801 commands the stage 150 to move to the startlocation and accelerate to the correct constant speed. Control computer2801 also commands illumination source 110 to emit light of the correctintensity to match the requirements of photosensitive substrate 140.This is usually done with a variable optical attenuator. Data from thestage metrology system 2802 tells the control computer when thesubstrate is in the correct position to begin exposure. Again referringto FIG. 19, at time T1 minus T/2, where T satisfies equation (1), thebottom of substrate array 1002 would be at substrate position ½. At thistime, the control computer commands the spatial light modulator toswitch all of the mirrors to the states corresponding to the new valuesstored in the SLM memory 2808. At the same time the control computer2801 commands the fast disk drives 2806 to send the next row of data tothe decompression electronics 2807, which loads the second frame ofmirror state data into the SLM memory. This process is repeated untilthe edge of the substrate is reached, at which time the control computercommands the stage to execute a turn-around; the system is then ready tostart exposing the next segment of the serpentine path, as shown in FIG.8. This is repeated until the entire patterned area of the substrate hasbeen exposed.

The method of operation discussed above with reference to FIG. 28 canreadily be extended to operate an optical lithography system of theinvention comprising multiple SLM area arrays.

Some embodiments of the optical lithography tool have an SLM withmultiple area arrays which are arranged in multiple rows, where all ofthe following apply:(1) the rows of area arrays are perpendicular to thedirection of movement of the projected image of the SLM arrays on thesubstrate; (2) the area arrays are individually aligned so that the rowsof elements in the arrays are also perpendicular to the direction ofmovement of the projected image of the SLM arrays on the substrate; and(3) the positions of the area arrays are staggered from one row to thenext. An example of such an arrangement is shown in FIG. 29. In FIG. 29the area arrays 2910 are arranged in three rows, where the rows areperpendicular to the direction of movement 2950 of the projected imageof the SLM arrays on the substrate (direction 2950 is also the directionin which pattern data is scrolled across the elements of the areaarrays). The arrangement of the SLM area arrays shown in FIG. 29 allowsa substrate to be exposed without having to follow a serpentine path asshown in FIG. 9 (the path in FIG. 9 is suitable for a single row of SLMarea arrays in which there will be gaps between arrays). The staggeredarrangement allows the gaps between the arrays in one row to be coveredby arrays in other rows. The example shown in FIG. 29 shows coveragewithout gaps where there is no overlap of coverage in the 3 rows;however, some embodiments may have overlap in coverage. Furthermore thearrangement of SLM area arrays within a roughly circular area (indicatedby circle 2960 in FIG. 29) makes efficient use of the imaging optics,which will typically consist of circular components. For example animage of the seven SLM arrays in FIG. 29 can all be simultaneouslyprojected onto a substrate by a projection lens system comprising asingle set of circular lenses.

FIG. 30 shows the optical lithography tool of FIG. 4 with the additionof a mirror 485, a light switching mechanism 121 and a second SLM beamdump 481. In this example the light switching mechanism 121 is a secondSLM. A light path from a light source (comprising components 410 through417) to a substrate 140, via a SLM 120 is indicated by light rays 170.The light switching mechanism 121 is positioned in serial with the SLM120 on the light path. In this example, a mirror 485 has also beeninserted on the light path to accommodate the SLM 121 in the positionshown. Clearly, many other optical configurations are possible that willaccommodate the light switching mechanism on the light path between thelight source and the SLM 120. The SLM 121 is a mirror array with mirrorsthat have two states—an “on” state in which the light is reflectedtoward the SLM 120, and an “off” state in which the mirror reflectslight toward second SLM beam dump 481. In this example all of themirrors are switched as one. A discussion of most of the components ofthe tool in FIG. 30 can be found in the text relating to FIG. 4. Furtherexplanation of the operation of the tool is given with reference to FIG.31.

In FIG. 31, the timing of the switching of the SLMs 120 and 121 is shownby waveforms 3120 and 3121, respectively. When SLM 120 is in the “on”state, all of the elements of the SLM may be individually “on” or “off”,in other words an exposure pattern may be loaded on the SLM. When theSLM 120 is in the “off” state, all of the elements of the SLM are “off”.The same is true for SLM 121, except all of the elements of the SLM are“on” when SLM 121 is in the “on” state. SLMs 120 and 121 have the sametime interval T between switching, in other words the same switchingfrequency; however, they are shifted out of phase by a time shift ofT(1−1/n). All elements of both SLMs are switched “off” every other timeinterval. Only when both SLMs are in the “on” state can light reach thesubstrate, which is for a time span T/n every other time interval.During this time span the projected image must move across the surfaceof the substrate a distance of one projected mirror pitch pM (which isthe same as one pixel's length on the substrate surface). This resultsin a stage speed v, given by the equation:

v=npM/T   (7)

where n is a constant. The time between exposures of the substrate is2T, during which time the pattern on the SLM 120 will have shifted by 2nrows. In principle n can have any value greater than 1; however,practical choices for n will typically be integers greater than one andless than 10.

FIG. 32 illustrates the shifting of patterns on the SLM and thecorresponding image on the substrate. In this example, the substrate ison a stage and moves at constant speed in the x direction duringexposures. The following are shown, with reference also to FIG. 30: partof SLM 120, which is an array of elements 3200 with an area of 12 rowsby 6 columns; a corresponding part of substrate 140, which is an arrayof pixels 3202 with an area of 4 rows by 6 columns; resultant image 3207with projected row pitch (width of a pixel) 1008. The resultant imageshows one possible latent image on the substrate due to completion ofthe entire series of exposures. “Snapshots” of the corresponding partsof the SLM and substrate are shown at equally spaced times T1 throughT7, where the time interval is T (the times T1 through T5 are alsolabeled in the timing diagram, FIG. 31, for reference). The parts of theSLM and substrate are indicated in FIG. 32 by M and S, respectively. TheSLM array 3200, the substrate array 3202 and the resultant image 3207are drawn as if viewed from a position directly above them and lookingdown in the −z direction of stationary coordinate system 160. For easeof illustration, in each “snapshot” the SLM and substrate arrays areshown next to each other. The projected row pitch 1008 in the resultantimage is the row pitch in the SLM array 3200 times the magnification ofthe projection lens system 430. However, for ease of illustration, ineach “snapshot” the SLM and substrate arrays are shown having the samesize and orientation. The grid shown on the arrays 3200 and 3202, andthe image 3207 is for reference only. A light square in 3200 correspondsto an SLM element in the “on” state, while a dark square corresponds toone in the “off” state. The light and dark areas in 3202 correspond tothe states of the SLM elements for that “snapshot”. The example shown inFIG. 32 is for n=2. An exposure is made every 2T and the pattern on theSLM array is seen to have moved by 4 rows during this time period. Theresultant image is the same as seen in FIG. 10, even though thesubstrate in FIG. 32 was moving twice as fast during exposure.

The approach described above with reference to FIGS. 30 through 32 is anexample of how to increase the throughput of substrates, without havingto reduce the switching time of the SLM. This is important when theminimum switching time for the SLM is already being used, since thethroughput of substrates can still be increased. The cost of thisincrease in throughput is a more complex lithography tool, including alight switching mechanism and an SLM with a larger number of rows (toaccommodate the movement of 2n rows between exposures).

Clearly, the tool of FIG. 30 can be used to implement gray leveltechniques, as described previously. The tool of FIG. 30 can be modifiedand operated in many ways, as described earlier for the tools of FIGS. 1through 6. For example a variety of image movement mechanisms, as shownin FIGS. 2 and 3, can be integrated into the tool of FIG. 30.

The light switching mechanism 121 in FIG. 30 can clearly be effective indifferent positions, both in front of and beyond the SLM 120 on thelight path, providing appropriate optical adjustments are made. Thelight switching mechanism may be integrated into the light source, andmay even be an intrinsic property of the light source (for example apulsed laser). The light switching mechanism can be a SLM, a shutter, arotating mirror, or any other optical component capable of controllingthe passage of light along the light path. Those skilled in the art willbe aware of the many ways in which these light switching mechanisms canbe incorporated and used in the many embodiments of the opticallithography tool of the invention. For example, the addition of somelenses between the SLM 121 and SLM 120 shown in FIG. 30 would allow animage of the pixels of SLM 121 to be focused, in one-to-onecorrespondence, onto the pixels of SLM 120—this would allow the SLM 121to be used to control the passage of light independently for differentblocks of array elements or even to control the passage of light on anindividual element basis.

Now to consider the case in which the light switching mechanism can beswitched faster than the SLM 120. In FIG. 33, the timing of theswitching of the SLM 120 and the light switching mechanism is shown bywaveforms 3320 and 3321, respectively. When SLM 120 is in the “on”state, all of the elements of the SLM may be individually “on” or “off”,in other words an exposure pattern may be loaded on the SLM. When theSLM 120 is in the “off” state, all of the elements of the SLM are “off”.The pattern on the SLM can be switched every time interval T The lightswitching mechanism is configured as a simple two state “on”/“off”switch. Only when both the SLM and the light switching mechanism are inthe “on” state can light reach the substrate. The light switchingmechanism provides the limiting time span Tin during which light canreach the substrate. During this time span the projected image must moveacross the surface of the substrate a distance of one projected mirrorpitch pM (which is the same as one pixel's length on the substratesurface). This results in a stage speed v, given by equation (7). Thetime between exposures of the substrate is T, during which time thepattern on the SLM 120 will have shifted by n rows. In principle n canhave any value greater than 1; however, practical choices for n willtypically be integers greater than one and less than 20, in which casethe time span will be a submultiple of said switching time interval.

FIG. 34 shows an optical lithography tool with optics configured toallow the projected images from two SLM area arrays, 3420 and 3421, tooverlap on the surface of a substrate. If desired, the overlappingimages may be brought into register—superimposed exactly, pixel forpixel. A light source 110 and prisms 3410 through 3413 provideillumination to two SLM area arrays 3420 and 3421. The light reflectedfrom the SLM area arrays is combined by prisms 3410 through 3413 andthen projected by imaging optics 3430 onto the photosensitive surface ofa substrate 140. The substrate 140 is carried by a stage 150 which movesthe substrate in the x-y plane of coordinate axes 160. The opticalconfiguration of FIG. 34 may be modified to include more SLM areaarrays. An example of an optical configuration allowing for theprojected images of three SLM area arrays to overlap on the surface of asubstrate is shown in U.S. Pat. No. 6,582,080 to Gibbon et al.,incorporated by reference herein. Those skilled in the art willappreciate that the tool shown in FIG. 34 may be modified along thelines of the apparatus shown in FIGS. 1 through 6, thus providing manyfurther embodiments of the invention. The apparatus of FIG. 34 can beoperated in a similar manner to that of FIG. 30. Further explanation ofthe operation of the tool is given with reference to FIG. 35.

In FIG. 35, the timing of the switching of the area arrays 3420 and 3421is shown by waveforms 3520 and 3521, respectively. When array 3420 is inthe “on” state, all of the elements of the array may be individually“on” or “off”, in other words an exposure pattern may be loaded on thearray. When the array 3420 is in the “off” state, all of the elements ofthe array are “off”. The same is true for array 3421. Arrays 3420 and3421 have the same time interval T between switching, in other words thesame switching frequency; however, they are shifted out of phase by atime shift of T(1−1/n). All elements of both arrays are switched “off”every other time interval. Both area arrays are in the “on” state and adouble dose of light reaches the substrate for a time span T/n everyother time interval. During this time span the projected image must moveacross the surface of the substrate a distance of one projected mirrorpitch pM (which is the same as one pixel's length on the substratesurface). This results in a stage speed v, given by equation (7). Thetime between double dose exposures of the substrate is 2 T, during whichtime the pattern on the SLM 120 will have shifted by 2n rows. Adjustmentof the dose and development conditions of the photosensitive surface ofthe substrate are made to ensure that only pixels which have receivedsufficient double dose exposures will form the developed pattern.

Clearly, the tool of FIG. 34 can be used to implement gray leveltechniques, as described previously. The tool of FIG. 34 can be modifiedand operated in many ways, as described earlier for the tools of FIGS. 1through 6 and 30. For example a variety of image movement mechanisms, asshown in FIGS. 2 and 3, can be integrated into the tool of FIG. 34.

An alternative mode of operation for the optical lithography tool ofFIG. 34 is to have the area arrays 3420 and 3421 operating in phase. Inthis case the speed of the substrate will be limited by equation (1).This mode of operation may be useful when a single area array is unableto deliver a large enough dose per unit time.

Referring to the description of FIGS. 8 and 9, an SLM area array isoriented in such a way that the columns of pixels in the projected imageon the substrate are parallel to the direction of movement of the imageitself. This results in blurring of the edges of the pixels which areorthogonal to the direction of movement; however, the edges parallel tothe direction of movement are not blurred by the movement. In order toimplement gray level techniques, the edges parallel to the direction ofmovement must also be blurred. Blurring of the parallel edges can beachieved in many ways as described earlier, all of which involveprojecting a blurred image of the SLM onto the substrate surface. Thereis an alternative approach to achieving blurred edges which can be usedwith all of the embodiments of the optical lithography tool disclosedabove—the SLM area array is oriented in such a way that the columns ofpixels in the projected image on the substrate are not parallel to thedirection of movement of the image itself. For example, the columns inthe projected image may be at an angle of 45 degrees to the direction ofmovement, in which case all of the edges of the square pixels will beequally blurred due to the movement alone.

While the invention has been described with reference to particularembodiments, this description is solely for the purpose of illustrationand is not to be construed as limiting the scope of the inventionclaimed below.

1-79. (canceled)
 80. A lithographic method, comprising the steps of: continuously illuminating a spatial light modulator, said spatial light modulator comprising an area array of individually switchable elements, wherein said elements of said spatial light modulator are arranged in rows and columns on a rectangular grid; moving a substrate relative to said spatial light modulator, said substrate having a photosensitive coating, wherein the direction of movement of said substrate is neither parallel nor orthogonal to said columns of said elements of said spatial light modulator; while said substrate is moving, projecting an image of said spatial light modulator on said photosensitive coating, said photosensitive coating comprising an area array of pixels, the size and configuration of said pixels corresponding to the projected spacing and configuration of individually switchable elements in said spatial light modulator; and while said substrate is moving, switching said elements of said spatial light modulator, said switching being between at least two states, wherein a first state allows light to reach said substrate and a second state stops light from reaching said substrate, said switching being controlled to define features in said photosensitive coating, whereby, relative to said photosensitive coating, said image of said spatial light modulator moves across said photosensitive coating and a pixel in said photosensitive coating receives, in serial, doses of energy from multiple elements of said spatial light modulator; wherein switching of said elements of said spatial light modulator is controlled to vary the number of elements of said spatial light modulator that serially contribute to the total dose of energy received by any pixel in said photosensitive coating, whereby some pixels in said photosensitive coating receive different, non-zero, total doses of energy.
 81. A lithographic method as in claim 80, wherein said direction of movement of said substrate is at 45 degrees to said columns of said elements of said spatial light modulator.
 82. A lithographic method as in claim 81, wherein said moving is controlled so that feature edges in said photosensitive coating are shifted by a sub-pixel distance from the pixel edges defined by said area array in said photosensitive coating.
 83. A lithographic method as in claim 80, wherein said illuminating step is implemented by a lamp system comprising an arc lamp.
 84. A lithographic method as in claim 83, further comprising the step of improving illumination uniformity.
 85. A lithographic method as in claim 84, wherein said improving step is implemented by a light pipe positioned between said arc lamp and said spatial light modulator.
 86. A lithographic method as in claim 84, wherein said improving step is implemented by a fly's eye lens array positioned between said arc lamp and said spatial light modulator.
 87. A lithographic method as in claim 80, wherein said illuminating step is implemented by a quasi-continuous laser.
 88. A lithographic method as in claim 80, further comprising, while said stage is moving, keeping the distance between said photosensitive coating and said projection optics constant, whereby said image is focused on said photosensitive coating.
 89. A lithographic method as in claim 80, wherein said switching of said elements of said spatial light modulator is controlled to compensate for non-uniform illumination of said spatial light modulator.
 90. A lithographic method as in claim 80, wherein said projecting step is implemented by projection optics and wherein said switching of said elements of said spatial light modulator is controlled to compensate for distortion and aberrations in said projection optics.
 91. A lithographic method as in claim 80, wherein said switching of said elements of said spatial light modulator is controlled to compensate for misalignment and distortion of said substrate.
 92. A lithographic method as in claim 80, wherein said correspondence of said pixels to said individually switchable elements in said spatial light modulator is one-to-one.
 93. A method as in claim 80, further comprising, prior to said projecting step: measuring the distortion of said substrate; and modifying a design data file to account for said substrate distortion; wherein said design data file determines the position of said features defined in said photosensitive coating.
 94. A method as in claim 80, further comprising, before said projecting step, aligning said substrate to projection optics, wherein said projecting of said image is implemented by said projection optics.
 95. A method as in claim 94, wherein said aligning step comprises comparing arbitrary features on said substrate to images previously stored in a memory device.
 96. A method as in claim 94, wherein said aligning step comprises comparing arbitrary features on said substrate to idealized images previously stored in a memory device.
 97. A method as in claim 96, wherein said idealized images are selected from the group consisting of a cross and a circle.
 98. A lithographic method, comprising the steps of: continuously illuminating a spatial light modulator, said spatial light modulator comprising an area array of individually switchable elements; moving a substrate relative to said spatial light modulator, said substrate having a photosensitive coating; while said substrate is moving, projecting an image of said spatial light modulator on said photosensitive coating, said photosensitive coating comprising an area array of pixels, the size and configuration of said pixels corresponding to the projected spacing and configuration of individually switchable elements in said spatial light modulator; while said substrate is moving, switching said elements of said spatial light modulator, said switching being between at least two states, wherein a first state allows light to reach said substrate and a second state stops light from reaching said substrate, said switching being controlled to define features in said photosensitive coating, whereby, relative to said photosensitive coating, said image of said spatial light modulator moves across said photosensitive coating and a pixel in said photosensitive coating receives, in serial, doses of energy from multiple elements of said spatial light modulator; and blurring said image; wherein switching of said elements of said spatial light modulator is controlled to vary the number of elements of said spatial light modulator that serially contribute to the total dose of energy received by any pixel in said photosensitive coating, whereby some pixels in said photosensitive coating receive different, non-zero, total doses of energy.
 99. A lithographic method as in claim 98, wherein said projecting step is implemented by projection optics and wherein said blurring step is implemented by moving said photosensitive coating to a point of desired defocus along the optic axis of said projection optics.
 100. A lithographic method as in claim 99, further comprising, while said stage is moving, keeping the distance between said photosensitive coating and said projection optics constant, whereby said desired defocus is maintained.
 101. A lithographic method as in claim 98, wherein said blurring step is implemented by a diffuser positioned between said spatial light modulator and said photosensitive coating.
 102. A lithographic method as in claim 98, wherein the widths of the instantaneous edge dose profiles of said features in said photosensitive coating are approximately equal to the widths of the corresponding resultant edge dose profiles.
 103. A lithographic method as in claim 102, wherein the widths of said instantaneous edge dose profiles and said resultant edge dose profiles are measured from the 75% light intensity to the 25% light intensity points.
 104. A lithographic method as in claim 98, wherein the instantaneous edge dose profiles of said features in said photosensitive coating are approximately Gaussian in shape. 